1. Field of the Invention
The present invention relates to a Group III nitride semiconductor light-emitting device and to a method for producing the same. More particularly, the present invention relates to a Group III nitride semiconductor light-emitting device in which strain applied to the light-emitting layer is relaxed, and to a method for producing the same.
2. Background Art
Generally, Group III nitride semiconductor light-emitting devices are produced through epitaxial growth of a Group III nitride semiconductor from a growth substrate. In this process, a plurality of Group III nitride semiconductor layers having different lattice constants are formed. Due to the lattice constant difference, strain (i.e., stress) generates in a relevant semiconductor layer. The stress generates a piezoelectric field, which deforms the potential of the quantum well of the light-emitting layer, thereby spatially separating electrons from holes. As a result, the probability of recombination between electrons and holes in the light-emitting layer decreases. In this case, light emission efficiency of the semiconductor light-emitting device drops.
In order to suppress, to a maximum possible extent, the effect of the strain generated in a semiconductor layer on the light-emitting layer, some techniques have been developed. One technique is a superlattice layer for relaxing the strain. The superlattice layer has two or more layer units having different lattice constants, whereby the strain applied to the light-emitting layer is relaxed. Another layer for relaxing the strain is a layer for preventing electrostatic breakdown of a semiconductor layer (hereinafter such a layer may be referred to as an “electrostatic-breakdown-voltage-improving layer”). Patent Document 1 discloses a technique of forming pits in an electrostatic-breakdown-voltage-improving layer (see, for example, paragraphs [0007] to [0010] of Patent Document 1). The electrostatic-breakdown-voltage-improving layer can prevent electrostatic breakdown of a semiconductor layer and can relax biaxial stress by virtue of the pits.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2007-180495
However, even when a superlattice layer is simply combined with an electrostatic-breakdown-voltage-improving layer, the two layers being formed so as to relax the strain to a maximum extent, the effect of relaxing the strain cannot be attained to a maximum extent. Instead, the light emission intensity of the semiconductor light-emitting device may be reduced in some cases. This is conceivably because the strain relaxation mechanism of the superlattice layer differs from that of the electrostatic-breakdown-voltage-improving layer. Therefore, strain relaxation is preferably realized by not only provision of a superlattice layer and an electrostatic-breakdown-voltage-improving layer in combination but also additional means.